1. Field of the Invention
The present invention relates to a multipurpose tyre (all-season or all-road tyre) for a motor vehicle.
More specifically, the invention relates to the grooving of a multipurpose tyre, in as much as it is particularly suitable for motor car tyres designed to give good performance both on dry roads and on wet roads, and even on snow-covered roads.
2. Description of the Related Art
As is known, in tyres of multipurpose type the pattern and the compound of the tread band are designed so as to satisfy the various demands specifically required with reference to the type of behaviour which the tyre needs to display on various types of surface, i.e. not only on dry or wet surfaces but also on snow-covered surfaces, for which its use is envisaged.
These requirements are mainly conflicting, as a result of which the tyres which have been produced to date normally represent a compromise between these requirements, but do not in any case, however, reach the performance levels obtained with a tyre specifically designed for use on a given type of road surface, whether this is dry, wet or snow-covered.
Generally speaking, irrespective of the use for which it is designed, a good tyre should have, inter alia, good properties of directionality and tractional power, as well as good wear resistance.
The directionality properties, which consist of the ability of the tyre to accurately maintain the set trajectory, are positively influenced by the presence of longitudinal grooves, while the tractional power properties, which consist of the capacity of the tyre to transmit tangential forces both during acceleration and when braking, are enhanced by providing grooves aligned transversely to the direction of motion. The efficacy of these transverse grooves as regards the tractional power is proportionately better the more the alignment of the grooves approaches a direction parallel to the axis of rotation of the tyre, so that, normally, angles of between 45° and 90° relative to an equatorial plane of the tyre are chosen for these grooves.
That being said, it should be pointed out that as regards tyres specifically designed for driving on dry surfaces, the number and width of the longitudinal and transverse grooves tend to be reduced to improve the wear resistance, the noise level in motion and the smooth ride provided by the tyre.
However, as regards driving on a wet surface, a larger groove width is desired, especially as regards the circumferential grooves, which are responsible for discharging the water collected under the imprint area of the tyre in order to avoid the occurrence of the well-known and hazardous aquaplaning effect.
Finally, as regards driving on snow-covered surfaces, a large number of small notches or grooves is required in order to adequately trap the snow picked up off the road surface, the snow-on-snow friction being greater than the rubber-on-snow friction.
From the foregoing, it is quite easy to appreciate the difficulties encountered in producing a multipurpose tyre which simultaneously has to satisfy all these conflicting requirements, arising specifically with regard to driving on dry, wet and snow-covered surfaces.
Specifically, the use of wide circumferential grooves conflicts both with the requirements of driving on dry surfaces, which prefer small grooves in order to increase the solid portion of the tread pattern, so as to improve the wear resistance and the noise level in motion, and with the requirements of driving on snow-covered surfaces, with regard to which wide grooves, which aid the self-cleaning of the tread pattern, produce a limitation of the behavioral properties, particularly tractional power and road holding.
The many notches required for use on snow-covered surfaces also conflict with the requirements for driving on dry surfaces, in which an excessive number of notches entails greater deformability of the blocks, with an increase in the noise level, and with the wear, as well as a reduction in the driving stability and smoothness of ride.
Thus it follows that the current multipurpose tyres have quite good hold under wet conditions, although this is inferior to that of modern tyres specifically designed for driving on wet surfaces, quite good tractional power in snowy conditions, but inferior to that which can be achieved by tyres specifically designed for winter conditions, as well as acceptable wear resistance and a sufficient hold on dry roads, although not to the same level as the best summer tyres commercially available.